It is previously known to provide working machines of different kinds, which are provided with four or more rubber wheels to normally be driven on roads or in terrain, with railway wheel units in the forward and rearward ends to also allow driving of the working machine along a railway track. Thereby, the working machine is guided along the rails of the railway track by means of the railway wheels of the railway wheel units while at the same time the rubber wheels of the working machine bear against the upper surfaces of the rails and accomplish driving and braking of the working machine during travel along the railway track. One example of such a railway wheel unit is disclosed in SE 509478 in which a tractor loader or excavator loader has such a railway wheel unit mounted in its forward and rearward end, respectively.
However, there are some problems associated with such railway wheel units. For instance, the bearing pressure between the railway wheels and the rails has to be sufficient large such that the working machine doesn't derail. Moreover, the wear of the rubber wheels usually become comparatively large since their bearing surfaces against the upper sides of the rails become so small which deform the rubber wheels considerably in case the bearing pressure against the rails become too large. This wear can be reduced by transferring a relatively large part of the weight of the working machine from the rubber wheels to the railway wheels, which can be achieved by pressing the railway wheels harder against the rails by means of power generating means, e.g. hydraulic cylinders as in the embodiment according to said SE 509478. However, hereby it is important that the bearing pressure between the rubber wheels and the rails does not become too small since in such case the working machine will lose drive and brake power, which deteriorates the working capacity of the working machine and may create a potential dangerous situation if the working machine accidentally begins to move on the rails along the railway track. Since the working machine in addition normally also is equipped with some form of working tool, such as e.g. an excavating unit, a loading bucket or a work platform on a lifting arm, the bearing pressure between the rubber wheels and the rails as well as between the railway wheels and the rails can change during varying loading of the working tools. Moreover, the bearing pressure between the rubber wheels and the rails need normally be larger at low temperatures and/or precipitation to achieve satisfactory drive and brake power.